Last night it stopped working and must have shut down and I found it off this morning. I have run this PC 24/7 for over a month before and had no problems at all. This time it had been running for about 5 days. I can see no obvious faults on the motherboard or power supply board. The hard drive definitely works as i have it plugged into another pc at the moment. The RAM has also been fine though i have not tested it yet.

When I press the power button I get a split second of power and see the LED's come on the fans and they twitch as if to start and then nothing. I can repeatedly press the power button to achieve the same effect.

Where would you guys start with your diagnostics? I'm thinking power supply but cause it actually provides power (though briefly) I would assume it is ok. RAM is my next idea and that could be quite easily tested in my main PC. Motherboard's LED comes on when its plugged into the power suggesting that it is also OK. So i'm running out of ideas.

Tried resetting the BIOS and now when I plug in the power wire now it repeatedly tries to start up and then dies and then starts up again etc. I'm not pressing the on button and the wire isnt even connected to turn it on.